Inner drive for magnetic drive pump

ABSTRACT

An inner drive for a magnetic drive pump includes a magnet supported on a yoke. The inner drive is driven about an axis to pump a corrosive process fluid. The magnet and yoke are fully encapsulated during the molding process to completely surround the magnet and yoke in a protective plastic shell. A sleeve is arranged radially outwardly of the magnet to provide further protection. Backing rings are arranged on either side of the magnet. A bonding material joins the plastic shell to the backing rings and sleeve to prevent a space from forming beneath the plastic shell that would become filled with the process fluid once it has permeated the plastic shell. A protective coating is arranged on at least a portion of the magnet to further insulate the magnet from the process fluid.

BACKGROUND OF THE INVENTION

This application relates to a magnetic drive centrifugal pump.

Magnetic drive centrifugal pumps include a wet portion, which containsthe process fluid that is being pumped, and a dry portion having adrive, which provides power to the pump fluid. The dry portion isexposed only to the atmosphere surrounding the pump. In one typicalmagnetic drive design, an inner and outer drive are separated by acontainment shell, which prevents the pump fluid from escaping to theenvironment. The outer drive, which is usually driven by an electricmotor, is located in the dry portion and magnetically drives the innerdrive in the wet portion that is attached to a pump impeller. Sincemagnetic drive pumps are sealless, they are often selected to pump veryacidic or caustic process fluids, such as hydrochloric acid, nitric acidand sodium hypochlorite.

Both the outer and inner drives have a series of magnets mounted aroundtheir peripheries. Each magnet is synchronously coupled to a respectivemagnet that is of an opposite pole on the other drive. The attractionbetween the magnets results in a magnetic coupling between the twodrives causing the inner drive to rotate at the same speed of the outerdrive, which is driven by the motor. The inner and outer drives must belocated relatively close together for efficient power transmission,which requires a relatively small clearance to be maintained between thecontainment shell and each drive. In one example, the clearance isapproximately 0.060 inch.

In one type of magnetic drive pump, the inner drive magnets areprimarily protected from the corrosive process fluid by a chemicallyresistant plastic shell, which is typically injection molded around themagnets of the inner drive. Corrosive process fluid eventually permeatesthe plastic shell, thus attacking the underlying magnets. Once thecorrosive process fluid has permeated the plastic coating, the shellswells causing interference between the inner drive and the containmentshell and pump failure.

Therefore, what is needed is an inner drive that is more resistant toswelling once the process fluid has permeated the plastic shell.

SUMMARY OF THE INVENTION

The present invention provides a magnetic pumping element, such as aninner drive of a magnetic drive pump, that includes additionalprotections from corrosive process fluid. The inner drive includes ayoke with multiple magnets supported on the yoke. A protective coatingsurrounds at least a portion of the magnet, and in one example, extendspartially over the yoke. Typically, a metallic member, such as anickel-based alloy sleeve, is arranged proximate to the magnet. Aplastic shell is arranged proximate to the sleeve. In one example, theshell completely encapsulates the yoke and magnet as a result of themolding process so that further operations, such as plastic welding, arenot required to encapsulate the yoke and magnet.

A bonding material is arranged between the plastic shell and metallicsleeve, including backing rings, joining the plastic shell and metallicsleeve to one another. The bonding material prevents formation of acavity that can become filled with the corrosive process fluid once ithas permeated the shell. Additionally, the bonding material prevents theprocess fluid from reacting with the sleeve and from migrating betweenthe plastic shell and the metallic sleeve/backing rings and into thejoints and magnet areas

Accordingly, the present invention provides an inner drive that is moreresistant to swelling once the process fluid has permeated the plasticshell.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically depicting a magneticdrive centrifugal pump assembly.

FIG. 2 is a partial cross-sectional view of an integrated impeller andinner drive assembly.

FIG. 3 is a cross-sectional view of the inner drive shown in FIG. 2 andtaken along line 3-3.

FIG. 4 is an enlarged view of the area indicated by circle 4 in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A magnetic drive centrifugal pump assembly 10 is schematically shown inFIG. 1. The assembly 10 includes a motor 12 that drives a pump 14. Themotor 12 and pump 14 are supported by a frame 16. The motor 12 includesa drive shaft 18 that is coupled to a driven shaft 20 of the pump 14.

An outer drive 22 is supported by the driven shaft 20. The outer drive22 includes magnets mounted on a periphery of the outer drive formagnetically driving an inner drive 28, which supports magnets having anopposite pole of the magnets on the outer drive 22.

The pump 14 includes a housing 24 that supports the driven shaft 20 andouter drive 22 in a dry portion 26 of the pump 14. A pump case 34provides a wet portion 36 for holding the process fluid, which isseparated from the dry portion 26. The pump case 34 houses the innerdrive 28, which is coupled to an impeller 30. The impeller 30 rotatesabout a stationary shaft 32. The process fluid is pumped from an inlet38 to an outlet 40 by the impeller 30.

In the example shown in FIG. 2, the inner drive 28 and impeller 30 areformed in such a way so as to provide an integral, or separable,impeller and inner drive assembly 42. A typical inner drive 28 includesa yoke 44 that supports multiple magnets 46 about its outer periphery.The yoke 44 is typically constructed from a magnetic conductor, such asductile iron, to absorb the magnetic flux lines behind the magnets 46.Front and/or rear backing rings 48 are arranged on the yoke adjacent toeither side of the magnets 46. The backing rings 48 are typicallyconstructed from a non-magnetic material such as stainless steel so thatthey do not interrupt the magnetic flux lines on the working side of themagnets.

A sleeve 56 is arranged radially outboard of the magnets 46 to protectthe magnets 46 from process fluid. The sleeve 56 may be constructed froma nickel-based alloy such as Hastelloy or Inconel. The sleeve 56 may bea thin can that is pressed over the magnets 46. Alternatively, thesleeve 56 may be a machined enclosure that is integral with and extendsaxially from one of the backing rings 48.

A shell 60 is molded about the yoke 44, magnets 46, backing rings 48 andsleeve 56 to protect the components from the process fluid. The shell 60may be constructed from a fluoroplastic such as EthyleneTetrafluoroethylene (ETFE). Other melt processible fluoropolymers mayalso be used, such as Perfluoroalkoxy (PFA). The resins may also beglass or carbon fiber reinforced. Fibers in the range of 10-35%, forexample, may be used, and in one example, 20%.

In the prior art, only the shell 60 and sleeve 56 protected the magnets46 from the process fluid that permeated the shell 60. However,increased protection from the corrosive process fluid is desired. Tothis end, the inventive inner drive 28 also includes a powder coating 52arranged over the magnets 46. The powder coating 52 may extend from oneaxial end of the yoke 44 to the other end of the yoke 44 providing abarrier that seals the magnets 46 relative to the yoke 44. The powdercoating 52 is arranged between the backing rings 48 and the yoke 44, inthe example shown. Referring to FIG. 3, generous fillets 50, currentlymade using potting material 54, are provided in gaps 49 between themagnets 46. The fillets 50 provide a smooth transition between themagnets 46 and yoke 44, which creates a smooth, continuous coating thatis free of pits and cracks. Potting material 54, which is typically usedin inner drives, fills the rest of the gaps 49 between the magnets 46and sleeve 56 in order to prevent sleeve rupture as a result ofinjection molding.

One suitable powder coating is an epoxy polyester hybrid, which has alow cure temperature (250-275° F.). One example hybrid has approximately50% epoxy and 50% polyester. The powder coating preferable has goodadhesion, chip resistance, and chemical resistance. More than one coatmay be desirable. The coating must withstand the molding temperatures ofthe shell 60 (over 600° F.). A table of the properties of examplessuitable potting and powder coatings materials follows. Property Filletand Potting Material Powder Coating Product Name 3M epoxy 1 partadhesive 2214 HD Sherwin Williams Powdura Powder PMF Coating - EpoxyPolyester Hybrid Base Modified epoxy base Polyester (80%), epoxy (20%)Major Ingredients Epoxy resin, aluminum pigments, polyester and epoxysynthetic elastomer Adhesion ASTM D-3359 - No failure with 1/16″ squares(cross hatch) Environmental Resistance ASTM D-1002 - 1910 psi steeloverlap ASTM D-B117 - passes 500 hr min shear 365 days in 100% RH saltfog test Outgassing Minimal NA Flexibility See hardness and strengthdata ASTM D-522 - pass on ⅛″ mandrel bend Density 1.5 g/ml ImpactResistance ASTM D-2794 - 100 lbs direct & reversed - excellentperformance Viscosity >1,000,000 cps-Brookfield (paste). Powderconsistency prior to oven Heated to thin for potting fill bake Hardness85 Shore D hardness (approx) ASTM D-3363 (for thin coatings) - 2H Pencilhardness Ultimate Tensile Strength 10,000 psi Modulus of Elasticity750,000 Coeff. of Thermal Expansion (cured) 49 × 10 − 6 in/in/C. (0-80C.) Cure Temp or Coating Temp 2 hrs @ 225 F. cure temp 275 F. coatingtemp Steel T-Peel (ASTM D-1876) 50 lbs per inch of width

It has been discovered that the process fluid reacts with the sleeve 56once it has permeated the shell 60 resulting in salts and othercompounds that create a build up of solid material under the plasticshell 60. This build up of material often results in localized swellingof the shell 60 that leads to failure of the pump 14. Additionally,process fluid that has permeated the shell 60 may be subjected to apumping effect by the flexing of the shell 60. This agitation of processfluid that has permeated the shell 60 accelerates corrosion of thesleeve 56 and forces product into joints and magnet areas.

To address this problem, the inventive inner drive 28 also employs abonding interface between the sleeve 56 and any other potentiallyreactive material, such as the backing rings 48 and the shell 60. Thisprevents the formation of a cavity that can fill with solid material orprocess fluid.

The bonding interface 58 is provided by a suitable bonding materialcapable of joining the material of the shell 60 to the material of thesleeve 56 and/or backing rings 48. In one example, the bonding materialmay be a bonding primer that is a blend of a polymeric adhesive and afluoropolymer. The bonding primer, in one example, is stable up to 550°F. with negligible to zero out gassing. Two examples of suitableformulations are:

Formulation 1:

-   -   PelSeal PLV2100 VITON elastomer, 33% solids-13 grams    -   PelSeal accelerator no. 4-0.5 milliliters    -   DuPont ETFE powder 532-6210-4.5 grams

Formulation 2:

-   -   Methyl ethyl ketone-13 grams    -   PelSeal PLV2100 VITON elastomer, 33% solids-13 grams    -   PelSeal acceleration no. 4-0.5 milliliters    -   DuPont ETFE powder 532-6210-4.5 grams

Formulation 2 results in a lower viscosity, and is preferably sprayed onas opposed to application by brush or pad.

The yoke 44, magnets 46, backing rings 48, and sleeve 56 are typicallyassembled into a unit and the shell 60 molded about the unit. A typicalmolding process results in a void in a molding support region 62. Themolding support region 62 results from a support 64 used during themolding process that locates the unit in a desired position as the shellis molded about the unit. This void in the molding support region 62must be filled by a secondary fusing operation, such as plastic welding.The fusing creates a boundary interface where poor bonding between thebase material and weld material can exist. This frequently results in aweakened area, which can provide a premature leak path for the corrosiveprocess fluid to enter and attack the magnets 46.

The present invention utilizes a molding process resulting in shell 60,fully encapsulating the unit. The support 64, which may be multiplepins, are retracted at a desired time during the molding process so thatthe material forming the shell 60 fills the mold support region 62during molding. The formulations of plastic used for the shell 60 betterenable the flow fronts of material within the mold to quickly fill themolding support region once the supports 64 have retracted.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. Inparticular, the materials disclosed and their properties are exemplaryonly and are no way intended to limit the scope of the invention. Forthese and other reasons, the following claims should be studied todetermine the true scope and content of this invention.

1. A magnetic driving element comprising: a yoke; a magnet supported onthe yoke; and a protective coating surrounding at least a portion of themagnet.
 2. The magnetic driving element according to claim 1, comprisinga backing ring arranged radially outwardly of the yoke and axiallyadjacent to the magnet, the protective coating arranged between thebacking ring and yoke and between the backing ring and magnet.
 3. Themagnetic driving element according to claim 2, a backing ring isarranged on each of opposing sides of the magnet, the protective coatingextending between opposing ends of the yoke beneath each backing ring.4. The magnetic driving element according to claim 1, wherein multiplemagnets are arranged around an outer periphery of the yoke, the magnetsforming gaps circumferentially between one another, with a pottingmaterial providing a fillet in the gaps between the magnets.
 5. Themagnetic driving element according to claim 4, wherein a sleeve isarranged radially outwardly of the protective coating, and a plasticshell arranged radially outwardly of the sleeve, a potting materialarranged in the gaps between the fillets and the sleeve.
 6. The magneticdriving element according to claim 1, wherein the protective powdercoating is an epoxy blend.
 7. The magnetic driving element according toclaim 6, the epoxy blend is a mixture of epoxy and polyester.
 8. Amagnetic driving element comprising: a magnet; a metallic memberarranged proximate to the magnet; a plastic shell arranged proximate tothe sleeve; and a bonding material arranged between the plastic shelland metallic member joining the plastic shell and metallic member to oneanother.
 9. The magnetic driving element according to claim 8, whereinthe metallic member is a sleeve arranged radially outwardly of themagnet, the bonding material adjoining the sleeve and the plastic shell.10. The magnetic driving element according to claim 8, wherein themetallic member is a backing ring arranged axially from the magnet, thebonding material adjoining at least one of the sleeve and backing ringsand the plastic shell.
 11. The magnetic driving element according toclaim 8, wherein the bonding material is a polymeric adhesive andfluoropolymer mixture.
 12. The magnetic driving element according toclaim 11, wherein the fluoropolymer is provided by an ETFE powder.
 13. Amagnetic driving element comprising: a yoke; a magnet supported on theyoke; a plastic shell fully encapsulating the yoke and magnet with acontinuous layer of molded material.
 14. The magnetic driving elementaccording to claim 13, wherein an end of the yoke including a moldingsupport surface for receiving a molding support during plastic moldingof the shell.
 15. The magnetic driving element according to claim 14,wherein a protective coating covers at least a portion of the magnet andyoke, the protective coating extending to the molding support surface.16. The magnetic driving element according to claim 13, wherein theplastic shell contains at least one of an ETFE and PFA material.
 17. Themagnetic driving element according to claim 16, wherein at least one ofan ETFE and PFA materials contain reinforcing fibers.